1. Field of the Invention
The present invention generally relates to a method to characterize a material using ultrasound measuring devices. In particular, the present invention relates to detecting defects in a material by comparing the results of a mathematical model and an ultrasonic signal emitted during laser ultrasound testing.
2. Description of Prior Art
Ultrasound testing methods are non-invasive, generally non-destructive, techniques used to measure features of materials. These features may include layer thickness, cracks, delamination, voids, disbonds, foreign inclusions, fiber fractions, fiber orientation, and porosity. The features may influence a given material's qualities and performance in given applications. Each application places unique demands on the material's qualities including the need for differing strength, flexibility, thermal properties, cost, or ultraviolet radiation resistance. With the changing demands, more non-invasive, non-destructive testing of materials is being performed using techniques such as ultrasound testing.
Ultrasound testing includes transducer-induced, laser and plasma-initiated ultrasound. Transducer-induced ultrasound techniques use piezoelectric transducers to induce an ultrasonic signal in an object.
Laser ultrasound techniques use a laser pulse. When the laser pulse is directed at an object, it causes thermal expansion in a small region. This thermal expansion causes ultrasonic waves. These ultrasonic waves are then measured by a detector and converted into information about the features of the object. The laser pulse may be generated by several lasers including a ruby laser, a carbon laser, and a Nd:YAG laser.
In some cases, a higher laser-energy density can be used and some matter at the material surface is ablated. The recoil effect of the pulverized matter launches ultrasonic waves in the material. Similarly to the thermoelastic regime, this ablation regime produces ultrasonic waves that can be detected and converted into information about the features of the object.
Similar to the laser ultrasound, plasma-induced ultrasound causes thermal expansion initiated ultrasonic waves. Often, a laser generates the plasma by directing a pulse at a false target in proximity to the manufactured object. The plasma then hits the manufactured object, producing an ultrasonic wave.
The manufactured object may be composed of different materials including metal, polymer, composite, or ceramic materials. The detector may be one of several devices. For example, the detector may be a transducer on the surface of the object, a laser interferometer directed at the object, or a gas-coupled laser acoustic detector, to name a few.
Ultrasound techniques are applied in research as well as industrial settings. In research, ultrasound techniques are used to test new materials for desired features. The technique is used to seek defects in material that has undergone stress or environmental endurance testing. In an industrial setting, the technique is used during scheduled servicing or during manufacturing to inspect parts for defects. Aircraft, automobile and other commercial industries have shown increasing interest in these techniques.
However, one difficulty associated with ultrasound techniques is found in discerning information about the features of the object from the measured ultrasonic waves. Many of the objects are constructed from composite materials with multiple layers. As the waves traverse the material, they reflect off interfaces or defects, propagate at differing speeds within different layers and change amplitude. The measured signal is a complex compilation of these reflections, ultrasonic velocity differences and amplitude changes. More layers and differing materials add to the complexity. In general, an expert is required to discern relevant aspects of the complex ultrasound signal.
One approach used by experts is to determine which peaks within the signal signify a reflection off of the back surface of the object. The expert then looks for smaller peaks between the back surface reflection peaks to determine number of layers or other structural features. The distance between smaller peaks or the amplitude of these peaks yields information about the thickness of a layer, the composition of the layer, or the interface between layers.
By implication, ultrasound techniques require a great deal of expertise. This requirement limits the broad application of ultrasonic techniques in industrial settings and makes the technique expensive. Another problem is the amount of time associated with translating an ultrasound signal into understandable information about the features within the object. Long translation times lead to expensive labor costs and reduced numbers of tests.
As such, many ultrasound techniques suffer from difficulties associated with translating complex ultrasound signals. Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.